One of semiconductor memories integrated in LSI is a nonvolatile memory. It is a device which does not lose its stored data when the LSI is turned off. It is a very important device for widespread application of LSI.
With regards to a nonvolatile memory of a semiconductor device, there is a description of so-called floating gate memories or memories using an insulating film on pages from 496 to 506 of S. Sze, Physics of Semiconductor Devices, 2nd edition, A Wiley-Interscience Publication. As described therein, it is known that different from a floating gate memory, a memory capable of storing charges in the interface of insulating films stacked one after another or in a trap in the insulating film does not need the formation of a new conductive layer and the CMOS LSI process is suited for the formation of such a memory.
It is however difficult to realize a memory which stores charges in an insulating film, because it must have sufficient charge retaining properties while injecting and releasing charges. With a view to overcoming this drawback, there is a proposal that stored information is rewritten by injection of charges having a different sign instead of release of charges. A description of this action can be found on page 63 of Symposium on VLSI technology, 1997. In this structure, a polycrystalline silicon gate causing memory action and a gate for selecting a cell are separated. A similar description can be found in U.S. Pat. No. 5,969,383.
In this memory cell structure, two transistors having NMOS as a base and having a memory gate on the side of a select gate are “vertically stacked”. This structure is illustrated as an equivalent circuit in FIG. 1. The gate insulating film of the memory gate has a so-called MONOS structure (Metal-Oxide-Nitride-Oxide-Semiconductor (Silicon)), that is, a structure in which a silicon nitride film is sandwiched between silicon dioxide films. The gate insulating film of the select gate is a silicon dioxide film. This film and a gate insulating film of CMOS at a peripheral region or logic region can be used in common, which will be described later based on formation steps. A diffusion layer electrode is formed with the select gate and memory gate as a mask. This memory cell has four basic actions, that is, (1) writing, (2) erasing, (3) retention and (4) reading. These actions are typically called as described above, but “writing” and “erasing” can be called by another name. Although there are various different operation methods, a description will next be made with representative ones. Here, a memory cell formed as NMOS type will be described, but a PMOS type can also be formed similarly in principle.
(1) Upon writing, a positive potential is applied to a diffusion layer on a memory gate side, while a ground potential similar to that of a substrate is applied to a diffusion layer on a select gate. A high gate over drive voltage is applied to the memory gate to turn on a channel below the memory gate. The channel is turned on by setting the potential of the select gate higher, for example, by 0.1 or 0.2V than a threshold voltage. The strongest electric field then appears near the boundary between two gates and many hot electrons are generated and injected into the memory gate side. This phenomenon is known as a source side injection (SSI) and a description on it can be found on pages 584 to 587 of A. T. Wu, et al., IEEE International Electron Device Meeting, Technical Digest, 1986. A floating gate memory cell is used in the above description, but an injection mechanism is the same in an insulating film memory cell. Hot electron injection by this system is characterized by that electric field concentration onto a boundary between a select gate and a memory gate occurs, resulting in concentrated injection to the memory gate at the side end portion of the select gate. A floating gate memory cell has a charge retention layer composed of an electrode. In the insulating film type, on the other hand, charges are stored in an insulating film so that electrons are retained in a very narrow region.
(2) Upon erasing, a negative potential is applied to the memory gate, while a positive potential is applied to the diffusion layer on the memory gate side. Strong inversion is therefore caused in a region where the memory gate at the end of the diffusion layer and the diffusion layer are overlapped, whereby a band-band tunnel phenomenon occurs, leading to the formation of holes. A description on this band-band tunnel phenomenon can be found, for example, on pages 718 to 721 of T. Y. Chan, et al., IEEE International Electron Device Meeting, Technical Digest, 1987. In this memory cell, holes generated are accelerated in the channel direction, drawn by a bias of the memory gate and injected into an ONO film, whereby an erasing operation is conducted. In other words, a threshold voltage of the memory gate which has increased by the charges of electrons can be reduced by the charges of the holes thus injected.
(3) During retention, the charges are retained as charges of carriers injected into the insulating ONO film. Since the carrier movement in the insulating film is remarkably slow, they can be retained desirably even without applying a voltage to the electrode.
(4) During reading, the channel below the select gate is turned on by applying a positive potential to the diffusion layer on the side of the select gate and applying a positive potential to the select gate. Charge data retained can be read out as a current by giving an appropriate memory gate potential (that is, an intermediate potential between the threshold voltage under a written state and a threshold voltage under an erased state) permitting discrimination of a difference in the threshold voltage of the memory gate given by the written and erased states.
The memory cell having a split gate and using a hole injection according to the above-described related art features that in a hole injection state, the threshold voltage lower than the initial threshold voltage can be actualized owing to the charges of holes so that a large read current is available under an erased state. A fundamental circuit constitution of a memory cell is illustrated in FIG. 1. This memory cell has a select transistor 1 and a memory transistor 2 connected in series. Since this memory cell has MOSFET as a basic constitution, its characteristics undergo a change according to the scaling-down of the device. A current driving power can be improved by shortening the gate length or thinning the gate insulating film. The smaller the channel width, the smaller a current value. These parameters of the select gate as a conventional MOSFET can be scaled down. In the memory gate, however, in order to maintain write/erase characteristics or charge retaining properties, thickness of the gate insulating film or gate length cannot be reduced. When the memory cell is scaled down, channel-width reducing effects appear clearly. This leads to a problem of lowering of a current driving power.